1. Field of the Invention
This invention relates to an output device which supplies a current to a load.
2. Description of the Related Art
A regulator IC has a problem in that the response of the output gets worse when the load current is changed abruptly. In this context, the term “worse” means that the response becomes nonlinear. For example, in a case of a low power regulator IC, the gain of the amplifier is low and the response gets worse easily. If the response worsens, there may be a possibility that the actual output voltage of the regulator IC is lower than the required output voltage.
For example, FIG. 1 illustrates the composition of a regulator IC according to the related art. In a case of the regulator IC according to the related art in FIG. 1, if a gain of a voltage amplifier 16 is increased or a drive current of a transistor 21 for driving an output transistor 11 is steadily increased, a transient response characteristic of the output of the regulator IC can be improved. In this context, the term “improves” means that the transient response characteristic becomes more linear.
FIG. 2 illustrates the composition of another regulator IC according to the related art. As illustrated in FIG. 2, a voltage amplifier 22 and a transistor 20 are further arranged in the regulator IC illustrated in FIG. 1. In a case of the regulator IC illustrated in FIG. 2, the transient response characteristic of the output can be improved. Namely, a difference between a reference voltage Vref and a feedback voltage Vfb of an output voltage Vout is monitored by the voltage amplifier 22. Only during a transitional period in which the output voltage Vout is lowered due to an increase in the load current, the transistor 20 is turned on by the voltage amplifier 22, so that the transient response characteristic of the output can be improved.
Besides the circuits illustrated in FIG. 1 and FIG. 2, a constant voltage circuit adapted to realize a high speed response to an abrupt change in the load current is known. For example, refer to Japanese Laid-Open Patent Publication No. 2005-353037.
However, if the gain of the voltage amplifier 16 is increased or the drive current of the transistor 21 is steadily increased, in order to improve the transient response characteristic of the output, the current consumption will also be increased.
In the case of the regulator IC illustrated in FIG. 2, both the voltage amplifier 16 and the voltage amplifier 22 are provided to compare the direct current voltage, they are influenced by the input offset of each of the voltage amplifiers, and the current consumption and the transient response characteristic easily fluctuate.
If it is assumed that ΔV1 denotes a first deviation of the input offset of the voltage amplifier 16 and ΔV2 denotes a second deviation of the input offset of the voltage amplifier 22, the total input offset ΔV of the voltage amplifiers 16 and 22 in combination is represented by √(ΔV12+ΔV22) (or represented by the square root of the sum of the squared first and second deviations). Therefore, the deviation of the input offset in the case of FIG. 2 is as large as the deviation of the input offset of the case of FIG. 1 multiplied by √(1+ΔV22/ΔV12) (or multiplied by the square root of the sum of one and the squared ratio of the second deviation to the first deviation), and the operating point of the gate of the transistor 20 in the case of FIG. 2 easily fluctuates.
In the case of the constant voltage circuit disclosed in Japanese Laid-Open Patent Publication No. 2005-353037, a capacitor C3 is inserted and connected in series between an output terminal OUT of the constant voltage circuit and an input terminal of a differential amplifier circuit AMP2. Hence, it is necessary for this constant voltage circuit that an additional reference voltage generating circuit for inputting the bias voltage Vb1 to the other input terminal of the differential amplifier circuit AMP2 be connected further. Therefore, the current consumption of the whole constant voltage circuit will be increased due to the current consumption of the additional reference voltage generating circuit.